Titanium carbide (TiC) exhibits high hardness (7-10 RH, or 3200 Kg.sub.f /mm.sup.2 with 50 g load), a low coefficient of friction, a high melting point (3170.degree. C.) and a good resistance to corrosion. These properties as well as high thermal conductivity makes TiC particularly suitable for many applications such as parts in mining equipment (coal chutes, slurry conveyors, mill rolls), and in earth moving equipment (cutter blades), self sharpening edges, metal machining tools, etc.
TiC powders and coatings can be obtained in pure form or in a composite form. The composite is a dispersion of TiC particles in a metal matrix, the metals being preferably one or more selected from the group of Al, Co, Cr, Cu, Fe, Mo, and Ni.
TiC has thus far been mainly produced by directly reacting titanium or titanium dioxide with carbon in a resistance furnace or an induction furnace, usually under a protective atmosphere. Another TiC production method is a so-called auxiliary metal bath AMB) process, wherein iron or steel scrap is mixed with titanium, ferrotitanium or titanium oxide and melted in a graphite crucible, with an optional additional carbon charge, under vacuum or in hydrogen atmosphere. The TiC is then recovered by leaching the solidified melt. The method is described in U.S. Pat. No. 2,515,463.
Powdered TiC is obtained by grinding the product of a known TiC production method.
Composite TiC powders are usually produced for thermally sprayed composite coatings. They are obtained by combining previously produced pure TiC powders and a powdered metallic substance. Liquid phase processes giving metal-coated TiC powders include electrolytical and hydrometallurgical processes such as the Sherritt Gordon autoclave method. Composite powders can also be obtained by gas phase deposition and by various agglomeration techniques using organic binders and by micropelletizing methods such as dispersion spray drying. Thermal post-treatments such as sintering are often used to improve and stabilize the quality of the micropellets and other composite powders.
TiC composite parts are generally obtained by various powder metallurgy techniques such as hot pressing, hot isostatic pressing, infiltrating, sintering, etc., using TiC powders in combination with metallic powders or using composite TiC powders, or both.
TiC coatings can be obtained by chemical vapour deposition (CVD) processes such as plasma activated CVD, ion plating, sputtering, etc. Hard-facing processes such as carbide particle injection process by laser-melting, electric contact sinter bonding and plasma-transferred arc (PTA) processes are also used. Plasma spraying is the most commonly applied hard-facing technique for TiC or TiC-based composites. Plasma spraying of TiC and of TiC-based composites uses previously produced TiC powders, alone or mixed with metallic powders respectively.
None of the currently known processes can produce TiC-based composite materials containing a fine uniform dispersion of TiC in the preferred elements--Al, Co, Cr, Cu, Fe, Mo, Ni and alloys thereof. What can be produced thus far is a random mixture of TiC particles with a metallic substance. It is desirable that the size of TiC crystals, uniformly distributed throughout the metallic matrix, be fine (less than ca 20 .mu.m) to very fine (less than 1 .mu.m). The existing processes can produce relatively coarse TiC particles, much larger than 20 micrometers in size, which then require grinding for any particle size refinement.
Exemplary methods of preparing titanium carbide are also described in Canadian Patents No. 1,072049 (Perugini) and 894,138 (Swaney) and in U.S. Pat. No. 4,161,512 (Merzhanov et al.). However, none of these deals with the problem of making a composite TiC-based material with fine-sized TiC crystals, distributed uniformly in a metal matrix. The Merzhanov et al patent uses a so-called self-propagating synthesis (SHS) wherein the starting materials are carbon and titanium, both finely divided. The exothermic reaction therebetween produces relatively coarse titanium carbide with the particle size predominantly in the 10-60 micron range. The Canadian patents propose to react, in a plasma furnace, titanium or titanium halide with compounds of a halogen and carbon, or methane. The resulting titanium carbide, indeed very fine sized, is not in composite form.
Dallaire et al. (U.S. Pat. No. 4,673,550) proposed a somewhat different process for obtaining titanium boride-based composite materials. The patent does not refer to titanium carbide. The process comprises reacting mixtures of titanium alloys with boron or ferroboron by effecting an exothermic reaction therebetween or by heating or melting. Hard-facing techniques may be employed to form TiB.sub.2 composite overlays according to that process.
As mentioned hereinabove, composite TiC powders are until now produced, e.g., for thermally sprayed composite coating, by combining previously produced TiC powder and a powdered metallic substance selected from the above-defined group of metals. Such coatings using these TiC-based mixtures and produced by various hard-facing techniques still give coarse coating structures with the carbide unevenly dispersed throughout the coating matrix.
It should be borne in mind that TiC does not always appear in stoichiometric form with the atomic ratio 1:1. Due to crystalline defects, the carbon content in the compound may vary. Therefore, this compound may be represented more precisely as Ti.sub.x C.sub.y but for the purpose of this description, the formula TiC will be used throughout.